Safe-mode operation of an implantable medical device

ABSTRACT

A method, system, and apparatus for implementing a safe mode operation of an implantable medical system using impedance adjustment(s) are provided. A first impedance is provided to a lead. An indication of a possibility of a coupled energy is received. Based upon said indication, a second impedance associated with the lead to reduce the coupled energy is provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a related application to U.S. patent applicationSer. No. 11/186,261, entitled “Safe-Mode Implantable Medical Devices,”which is filed on the same date as the present application and in thename of the same inventors.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to implantable medical devices, and,more particularly, to methods, apparatus, and systems for providing asafe-mode operation of the implantable medical device using a dynamicimpedance adjustment process.

2. Description of the Related Art

There have been many improvements over the last several decades inmedical treatments for disorders of the nervous system, such as epilepsyand other motor disorders, and abnormal neural discharge disorders. Oneof the more recently available treatments involves the application of anelectrical signal to reduce various symptoms or effects caused by suchneural disorders. For example, electrical signals have been successfullyapplied at strategic locations in the human body to provide variousbenefits, including reducing occurrences of seizures and/or improving orameliorating other conditions. A particular example of such a treatmentregimen involves applying an electrical signal to the vagus nerve of thehuman body to reduce or eliminate epileptic seizures, as described inU.S. Pat. No. 4,702,254 to Dr. Jacob Zabara, which is herebyincorporated in its entirety herein by reference in this specification.Electrical stimulation of the vagus nerve (hereinafter referred to asvagus nerve stimulation therapy or VNS) may be provided by implanting anelectrical device underneath the skin of a patient and performing adetection and electrical stimulation process. Alternatively, the systemmay operate without a detection system once the patient has beendiagnosed with epilepsy, and may periodically apply a series ofelectrical pulses to the vagus (or other cranial) nerve intermittentlythroughout the day, or over another predetermined time interval.

Generally, therapeutic electrical stimulation is delivered by theimplantable device via a lead. The lead generally terminates onto anelectrode, which may be affixed onto a tissue. A plurality of electrodesthat are associated with an implantable medical device are generallyoperatively connected to the implantable device via individual leads.Therefore, a number of leads may project from the implantable deviceonto various portions of a patient's body. For example, a number ofelectrodes may be attached to various points of a nerve or other tissueinside a human body.

Occasionally, a patient having an implantable device may be subjected toan electrical field, a magnetic field, and/or an electromagnetic field.In the proximity of one of the aforementioned fields, coupled signaland/or noise may appear on various portions of the implantable devicesystem, particularly on the leads. Depending on the strength of thefield, a significant amount of coupled energy may appear on the leads.This coupled energy may cause adverse effects. For example, the coupledenergy on the leads may affect operation of the device, or cause adversethermal changes. The coupled signal or energy may also interfere withthe delivery of the electrical/magnetic stimulation therapy, or with theproper detection of various signals from the electrodes. Other adverseeffects, such as heating of various portions of the implantable systemmay occur. This heating may damage tissue that is proximate to theportion of the implantable system that experiences thermal the changes.

The present invention is directed to overcoming, or at least reducing,the effects of one or more of the problems set forth above.

SUMMARY OF THE INVENTION

In one aspect, the present invention comprises a method for implementinga safe mode operation of a medical device using impedance adjustment(s).A first impedance is provided to a lead. An indication of a possibilityof a coupled energy is received. Based upon said indication, a secondimpedance associated with the lead to reduce the coupled energy isprovided.

In another aspect, the method comprises a method for implementing a safemode operation of a medical device using impedance adjustment(s). Afirst impedance associated with a lead set of the implantable medicaldevice for performing the stimulation is provided. A coupled energy onthe lead set is detected. A second impedance associated with the leadset in response to detecting the coupled energy is provided.

In a further aspect, an additional method of implementing a safe modeoperation using impedance adjustment(s) is provided. Data indicating apotential presence of a field is received. The field is an electricalfield, a magnetic field, or an electromagnetic field. An impedanceassociated with a lead coupled to the IMD is modified based upon thedata indicating the potential presence of a field.

In another aspect of the present invention, an implantable medicaldevice is provided implementing a safe mode operation using impedanceadjustment(s). The implantable medical device includes a stimulationunit to provide a stimulation signal through a lead operatively coupledto the IMD. The implantable medical device also includes a controller toreceive an indication of a possibility of a coupled energy. Thecontroller is also adapted to modify an impedance of the lead set basedupon detection of the coupled energy.

In another aspect of the present invention, a medical device system isprovided for implementing a safe mode operation using impedanceadjustment(s). The system includes an electrode coupled to a tissue in apatient's body, and a lead operatively coupled to the electrode. Thelead is adapted to carry a stimulation signal to the electrode. Thesystem includes an implantable medical device (IMD) operatively coupledto the lead. The IMD, which may comprise a signal generator, is adaptedto provide a stimulation signal to the tissue through the lead. The IMDincludes a stimulation unit to provide a stimulation signal through thelead and a controller to receive an indication of a possibility of acoupled energy. The controller is adapted to also modify an impedance ofthe lead set based upon the indication of a possibility of a coupledenergy.

In yet another aspect, the present invention comprises a computerreadable program storage device encoded with instructions forimplementing a safe mode operation using impedance adjustment(s). Theinstructions in the computer readable program storage device, whenexecuted by a computer, perform a method comprising: providing a firstimpedance relating to the lead; receiving an indication of a possibilityof a coupled energy; determining a second impedance associated with thelead to reduce the coupled energy; and modifying the first impedance ofthe lead to the second impedance.

In another aspect of the present invention, a medical device system isprovided for implementing a safe mode operation using impedanceadjustment(s). The system includes an electrode coupled to a portion ofa tissue in a patient's body. The system also includes a leadoperatively coupled to the electrode. The lead is adapted to carry astimulation signal to the electrode. The system also includes animplantable medical device (IMD) operatively coupled to the lead. TheIMD, which may comprise a signal generator, is adapted to provide astimulation signal to the tissue through the lead. The IMD includes astimulation unit to provide a stimulation signal through the lead. TheIMD further comprises an impedance unit to modify an impedance of thelead based upon a command from an external source. The IMD mayoptionally comprise a signal detection unit to detect a coupled energy.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich like reference numerals identify like elements, and in which:

FIGS. 1A-1D provide stylized diagrams of an implantable medical deviceimplanted into a patient's body for providing stimulation to a portionof the patient's body, in accordance with one illustrative embodiment ofthe present invention;

FIG. 2 is a block diagram of an implantable medical device and anexternal unit that communicates with the implantable medical device, inaccordance with one illustrative embodiment of the present invention;

FIG. 3 is a more detailed block diagram depiction of a signal detectionunit of the implantable medical device of FIG. 2, in accordance with oneillustrative embodiment of the present invention;

FIG. 4 is a more detailed stylized depiction of an impedance unit of theimplantable medical device of FIG. 2, in accordance with oneillustrative embodiment of the present invention;

FIG. 5 is a stylized depiction of a schematic relating to variousimpedances between various points of an implantable medical devicesystem, in accordance with one illustrative embodiment of the presentinvention;

FIG. 6 illustrates a flowchart depiction of a method, in accordance withmultiple illustrative embodiments of the present invention; and

FIG. 7 illustrates a more detailed flowchart depiction of performing adynamic impedance adjustment process of FIG. 6, in accordance with oneillustrative embodiment of the present invention.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Illustrative embodiments of the invention are described herein. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. In the development of any such actualembodiment, numerous implementation-specific decisions must be made toachieve the design-specific goals, which will vary from oneimplementation to another. It will be appreciated that such adevelopment effort, while possibly complex and time-consuming, wouldnevertheless be a routine undertaking for persons of ordinary skill inthe art having the benefit of this disclosure.

FIGS. 1A-1D illustrate an implantable medical system 100 that is capableof entering a safe-mode operation in response to a presence of a coupledsignal/energy experienced by a component of the system 100. Thesafe-mode operation may involve adjusting an impedance associated withthe portion of the implantable medical system 100 that is experiencingthe presence of the coupled signal/energy. The system 100 is alsocapable of detecting when the coupled signal/energy has been removed orsubstantially reduced, and returning to a normal operating mode.

FIGS. 1A-1D depict a stylized implantable medical system 100 forimplementing one or more embodiments of the present invention. FIGS.1A-1D illustrate a signal generator 110 having main body 112 comprisinga case or shell 121 with an electrical connector 116 in a header 114(FIG. 1C) for connecting to leads 122. The signal generator 110 isimplanted in the patient's chest in a pocket or cavity formed by theimplanting surgeon just below the skin (indicated by a dotted line 145,FIG. 1B), similar to the implantation procedure for a pacemaker pulsegenerator.

A stimulating electrode assembly 125, preferably comprising an electrodepair, is conductively connected to the distal end of an insulatedelectrically conductive lead assembly 122, which preferably comprises apair of lead wires (one wire for each electrode of an electrode pair).Lead assembly 122 is attached at its proximal end to the electricalconnector 116 on header 114. The electrode assembly 125 is surgicallycoupled to the patient's tissue, e.g., a vagus nerve 127 in thepatient's neck. The present invention is suitable for use in implantablemedical devices connected to any body tissue, e.g., a pacemaker coupledto heart tissue. The electrode assembly 125 preferably comprises abipolar stimulating electrode pair (FIG. 1D), such as the electrode pairdescribed in U.S. Pat. No. 4,573,481 issued Mar. 4, 1986 to Bullara.Persons of skill in the art will appreciate that many electrode designscould be used in the present invention. For embodiments of the presentinvention involving vagus nerve stimulation, two electrodes arepreferably wrapped about the vagus nerve, and the electrode assembly 125is preferably secured to the nerve 127 by a spiral anchoring tether 128(FIG. 1D) such as that disclosed in U.S. Pat. No. 4,979,511 issued Dec.25, 1990 to Reese S. Terry, Jr. and assigned to the same assignee as theinstant application. Lead assembly 122 is secured, while retaining theability to flex with movement of the chest and neck, by a sutureconnection 130 to nearby tissue.

In one embodiment of the invention involving nerve stimulation, the openhelical design of the electrode assembly 125 (described in detail in theabove-cited Bullara patent), which is self-sizing and flexible,minimizes mechanical trauma to the nerve and allows body fluidinterchange with the nerve. The electrode assembly 125 preferablyconforms to the shape of the nerve, providing a low stimulationthreshold by allowing a large stimulation contact area with the nerve.Structurally, the electrode assembly 125 comprises two electrode ribbons(not shown), of a conductive material such as platinum, iridium,platinum-iridium alloys, and/or oxides of the foregoing. The electroderibbons are individually bonded to an inside surface of an elastomericbody portion of the two spiral electrodes 125-1 and 125-2 (FIG. 1D),which may comprise two spiral loops of a three-loop helical assembly.The elastomeric body portion of each loop preferably comprises siliconerubber, and the third loop 128 (which typically has no electrode) actsas the anchoring tether 128 for the electrode assembly 125.

The lead assembly 122 may comprise two distinct lead wires or a coaxialcable whose two conductive elements are respectively coupled to one ofthe conductive electrode ribbons 125-1 and 125-2. One suitable method ofcoupling the lead wires or cable to the electrodes comprises a spacerassembly such as that disclosed in U.S. Pat. No. 5,531,778, althoughother known coupling techniques may be used.

In certain embodiments of the invention, sensing elements may be used toprovide data to the implantable medical system 100 concerning one ormore body parameters. Although exemplary sensors are disclosed herein,persons of skill in the art will appreciate that the present inventionis not limited to particular embodiments. Referring to FIG. 1B, eyemovement sensing electrodes 133 may be implanted at or near an outerperiphery of each eye socket in a suitable location to sense musclemovement or actual eye movement. The electrodes 133 may be electricallyconnected to leads 134 implanted via a catheter or other suitable means(not shown) and extending along the jawline through the neck and chesttissue to the signal generator 110. When included in systems of thepresent invention, the sensing electrodes 133 may be utilized fordetecting rapid eye movement (REM) in a pattern indicative of a disorderto be treated, as described in greater detail below.

Alternatively or additionally, EEG sensing electrodes 136 may optionallybe implanted in spaced apart relation through the skull, and connectedto leads 137 implanted and extending along the scalp and temple and thento the signal generator 110 in the same manner as described above forthe eye movement electrode leads. Electrodes 133 and 136, or other typesof sensors, may be used in some embodiments of the invention to triggeradministration of the electrical stimulation therapy to the vagus nerve127 via electrode assembly 125. Use of such sensed body signals totrigger or initiate stimulation therapy is hereinafter referred to as a“feedback” or “active” stimulation. Other embodiments of the presentinvention utilize a stimulation therapy delivered according to aprogrammed on/off duty cycle without the use of sensors to triggertherapy delivery. This type of delivery may be referred to as “passive,”“non-feedback,” or prophylactic stimulation. Both active and passivestimulation may be combined or delivered by a single IMD according tothe present invention. Either or both modes may be appropriate to treatthe particular disorder diagnosed in the case of a specific patientunder observation. The therapeutic electrical signal may be a continuousor pulsed signal; either type of signal may be applied periodically orintermittently to the vagus nerve.

The signal generator 110 may be programmed with an external computer 150(FIG. 1A) using programming software of the type copyrighted by theassignee of the instant application with the Register of Copyrights,Library of Congress, or other suitable software based on the descriptionherein, and a programming wand 155 may be used to facilitate radiofrequency (RF) communication between the computer 150 and the signalgenerator 110. The wand 155 and software permit noninvasivecommunication with the generator 110 after the latter is implanted. Thewand 155 is preferably powered by internal batteries, and provided witha “power on” light to indicate sufficient power for communication.Another indicator light may be provided to show that data transmissionis occurring between the wand and the generator.

A wide variety of stimulation therapies may be provided in implantablemedical systems 100 of the present invention. Different types of nervefibers (e.g., A, B, and C fibers being different fibers being targetedfor stimulation) respond differently to stimulation from electricalsignals. More specifically, the different types of nerve fibers havedifferent conduction velocities and stimulation thresholds, andtherefore differ in their responsiveness to stimulation. Certain pulsesof an electrical stimulation signal, for example, may be below thestimulation threshold for a particular fiber and therefore may generateno action potential in the fiber. Thus, smaller or narrower pulses maybe used to avoid stimulation of certain nerve fibers (such as C fibers)and target other nerve fibers (such as A and/or B fibers, whichgenerally have lower stimulation thresholds and higher conductionvelocities than C fibers). Additionally, techniques such aspre-polarization may be employed wherein particular nerve regions may bepolarized before a more robust stimulation is delivered, which maybetter accommodate particular electrode materials. Furthermore, opposingpolarity phases separated by a zero current phase may be used to exciteparticular axons or postpone nerve fatigue during long-term stimulation.

Regardless of the type of stimulation employed, in preferred embodimentsof the present invention, the signal generator 110 is coupled to thestimulation electrodes by leads 122. In the presence of a significantelectromagnetic field, a coupled signal or energy may appear on theseleads. The leads may behave as antennas that initiate an energy gradienton its surface. This energy may interfere with operation of theimplantable medical system, and may also cause release of thermalenergy, leading to excessive heating of the surrounding tissue.Embodiments of the present invention provide for performing a dynamicadjustment of an impedance to reduce the effects of the coupledsignal/energy.

Embodiments of the present invention provide for reducing a coupledsignal/energy experienced by a portion (e.g., the leads) of animplantable medical system 100. When a patient experiences anelectromagnetic (or any type) of energy field, a coupled energy mayappear on a portion of an implantable system. For example, the leadsassociated with the device may experience a coupled energy that mayinterfere with various operations of the system 100. The coupled energymay interfere with delivery of stimulation signals provided by theimplantable system 100. The coupled energy may also interfere withdetection of a signal associated with a patient's body sensed by theimplantable device.

Additionally, the energy coupled onto the leads may cause a rise in, ora release of, thermal energy, which may burn or otherwise adverselyaffect a portion of adjacent tissue. Embodiments of the presentinvention provide for reducing the coupled energy, thereby preventing orreducing an unwarranted increase in thermal energy. For example,embodiments of the present invention provide for reducing the amount ofenergy that is coupled onto a portion (e.g., leads) of the implantablesystem.

An impedance associated with various portions of the implantable systemmay be modified to substantially reduce energy that is coupled onto aportion of the implantable system 100. For example, if a leadexperiences coupled energy, an impedance associated with that particularlead may be adjusted in an automated and/or in a manual fashion. Thisadjustment of the impedance may cause an attenuation of the coupledenergy. Therefore, the impedance between multiple electrodes, theimpedance between an electrode and the casing of the device, and/or theimpedance between any two points associated with the implantable system100 may be adjusted or modified according to the type of energy coupledto the medical system 100. Hence, based on the strength, frequency,and/or other characteristics of the coupled energy, one of a pluralityof impedance adjustments may be performed to substantially reduce thecoupled energy and/or its effects.

Turning now to FIG. 2, a block diagram depiction of an implantablemedical device (IMD), in accordance with one illustrative embodiment ofthe present invention is illustrated. The IMD 200 may be used forstimulation to treat various disorders, such as epilepsy, depression,bulimia, heart rhythm disorders, etc. The IMD 200 may be coupled tovarious leads associated with the leads 122 (FIG. 1A). Stimulationsignals used for therapy may be transmitted from the IMD 200 to targetareas of the patient's body, specifically to various electrodesassociated with the leads 122. Stimulation signals from the IMD 200 maybe transmitted via the leads 122 to stimulation electrodes associatedwith the electrode assembly 125 (FIG. 1A). Further, signals from sensorelectrodes, e.g., 133, 136 (FIG. 1B) associated with correspondingleads, e.g., 134, 137, may also traverse the leads back to the IMD 200.

The implantable medical device 200 may comprise a controller 210 capableof controlling various aspects of the operation of the IMD 200. Thecontroller 210 is capable of receiving internal data and/or externaldata and generating and delivering a stimulation signal to targettissues of the patient's body. For example, the controller 210 mayreceive manual instructions from an operator externally, or may performstimulation based on internal calculations and programming. Thecontroller 210 is capable of affecting substantially all functions ofthe IMD 200.

The controller 210 may comprise various components, such as a processor215, a memory 217, etc. The processor 215 may comprise one or moremicrocontrollers, microprocessors, etc., that are capable of performingvarious executions of software components. The memory 217 may comprisevarious memory portions where a number of types of data (e.g., internaldata, external data instructions, software codes, status data,diagnostic data, etc.) may be stored. The memory 217 may comprise randomaccess memory (RAM) dynamic random access memory (DRAM), electricallyerasable programmable read-only memory (EEPROM), flash memory, etc.

The IMD 200 may also comprise a stimulation unit 220. The stimulationunit 220 is capable of generating and delivering stimulation signals toone or more electrodes via leads. A number of leads 122, 134, 137 may becoupled to the IMD 200. Therapy may be delivered to the leads 122 by thestimulation unit 220 based upon instructions from the controller 210.The stimulation unit 220 may comprise various circuitry, such asstimulation signal generators, impedance control circuitry to controlthe impedance “seen” by the leads, and other circuitry that receivesinstructions relating to the type of stimulation to be performed. Thestimulation unit 220 is capable of delivering a controlled currentstimulation signal over the leads 122.

The IMD 200 may also comprise a power supply 230. The power supply 230may comprise a battery, voltage regulators, capacitors, etc., to providepower for the operation of the IMD 200, including delivering thestimulation signal. The power supply 230 comprises a power-sourcebattery that in some embodiments may be rechargeable. In otherembodiments, a non-rechargeable battery may be used. The power supply230 provides power for the operation of the IMD 200, includingelectronic operations and the stimulation function. The power supply230, may comprise a lithium/thionyl chloride cell or a lithium/carbonmonofluoride cell. Other battery types known in the art of implantablemedical devices may also be used.

The IMD 200 also comprises a communication unit 260 capable offacilitating communications between the IMD 200 and various devices. Inparticular, the communication unit 260 is capable of providingtransmission and reception of electronic signals to and from an externalunit 270. The external unit 270 may be a device that is capable ofprogramming various modules and stimulation parameters of the IMD 200.In one embodiment, the external unit 270 is a computer system that iscapable of executing a data-acquisition program. The external unit 270may be controlled by a healthcare provider, such as a physician, at abase station in, for example, a doctor's office. The external unit 270may be a computer, preferably a handheld computer or PDA, but mayalternatively comprise any other device that is capable of electroniccommunications and programming. The external unit 270 may downloadvarious parameters and program software into the IMD 200 for programmingthe operation of the implantable device. The external unit 270 may alsoreceive and upload various status conditions and other data from the IMD200. The communication unit 260 may be hardware, software, firmware,and/or any combination thereof. Communications between the external unit270 and the communication unit 260 may occur via a wireless or othertype of communication, illustrated generally by line 275 in FIG. 2.

The IMD 200 also comprises an impedance unit 250 and may optionallycomprise a signal detection unit 240. The signal detection unit 240, ifpresent, provides for detecting the presence of a signal/energy. Thoseskilled in the art would appreciate that concepts of embodiments of thepresent invention may be implemented without the use of the signaldetection unit 240. The signal detection unit 240 is capable ofdetecting a signal/energy that may be coupled onto any portion of theimplantable system 100 (e.g., the electrodes, the leads, and/or the IMD200). For example, a coupled energy, signal, and/or noise that arecoupled onto a lead associated with the IMD 200 may be detected by thesignal detection unit 240. A more detailed description of the signaldetection unit 240 is provided in FIG. 3 and the accompanyingdescription below.

The impedance unit 250 is capable of modifying the impedance relating toone or more portions of the implantable system. For example, theimpedance unit 250 may modify the impedance between one lead relative toanother, and/or the impedance between a lead relative to a nodeassociated with the IMD 200. A more detailed description of theimpedance unit 250 is provided in FIG. 5 and the accompanyingdescription below.

One or more blocks illustrated in the block diagram of IMD 200 in FIG. 2may comprise hardware units, software units, firmware units and/or anycombination thereof. Additionally, one or more blocks illustrated inFIG. 2 may be combined with other blocks, which may represent circuithardware units, software algorithms, etc. Additionally, any number ofthe circuitry or software units associated with the various blocksillustrated in FIG. 2 may be combined into a programmable device, suchas a field programmable gate array, an ASIC device, etc.

Turning now to FIG. 3, a more detailed block diagram illustration of thesignal detection unit 240 is provided. A number of methods may be usedby those skilled in the art having benefit of the present disclosure todetect a signal, energy, and/or noise that is coupled onto any portionof the implantable system 100. FIG. 3 illustrates exemplary systems todetect such coupled energy. However, those skilled in the art havingbenefit of the present disclosure would appreciate that a variety ofcircuits may be used to detect coupled energy, and remain within thespirit and scope of the present invention.

The signal detection unit 240 may comprise a frequency count unit 310.The frequency count unit 310 may comprise various circuit portions, suchas a frequency divider 315 and/or a frequency counter 317. The frequencycount unit 310 may be capable of detecting a particular type of coupledsignal/energy that is coupled onto a portion of the implantable system100. The frequency count unit 310 is also capable of dividing thefrequency and/or counting the number of periods associated with thefrequency of the coupled signal/energy. Data relating to the frequencydivision process and/or the frequency counting process may be used toperform comparison(s) with stored data. This comparison may be useful indetermining whether coupled energy rises to the level of requiring anaffirmative response by the IMD 200. In other words, the comparison maybe used to determine whether an adjustment of the impedance relating tothe portion of the IMD 200 experiencing the coupled energy, is requiredin order to reduce the effect(s) of the coupled energy.

Coupled signal/energy on a portion of the implantable system 100 maycause a resonant effect, which could cause adverse conditions, such asheating of the leads. Therefore, it may be desirable to reduce theresonant effects of a coupled signal/energy. In order to create ananti-resonant effect, a reduction of heating, or to counter otheradverse effects resulting from the coupled signal/energy, a look-upprocess may be performed by the IMD 200. Data relating to thecharacteristic(s) (e.g., frequency, amplitude, etc.,) of the coupledenergy may be used to look up a counter-coupling impedance that wouldreduce the adverse effect(s) of the coupled signal/energy. Thisimpedance may reduce the magnitude of the coupled energy. The IMD 200may look up impedance data relating to the detected frequency of thecoupled energy, find or determine a corresponding impedance that mayreduce the effect of the detected frequency, and implement suchimpedance.

The signal detection unit 240 may also comprise a bandpass filtercircuit 320. The bandpass filter circuit may comprise various filters,such as a highpass filter 325 and/or a lowpass filter 327. These filtersmay filter out various frequency ranges so that the coupledsignal/energy may be analyzed. This analysis may be used to determinethe type of impedance adjustment or response that may be desirable. Forexample, the bandpass filter 320 may perform filtering processes todetect the presence of a 1.5 Tesla and/or a 3.0 Tesla Magnetic ResonanceImaging (MRI) signal/energy. Other types of MRI signals may also bedetected by the IMD 200 using the bandpass filter circuit 320.

Further, the signal detection unit 240 may also comprise envelopecomparator circuitry 330. The envelope comparator circuitry 330 mayprovide comparison of the coupled energy/signal in a range of values toreference-voltage or reference-current signals in order to characterizethe coupled energy. Further, the signal detection unit 240 may alsocomprise a signal threshold detector 340 that is capable of detecting avoltage or current level threshold relating to the coupled signal orenergy. Additionally, other sensors, such as thermal sensors 350, may beencompassed within the signal detection unit 240. The thermal sensor350, for example, may detect the thermal energy on the leads that may becaused by coupled energy.

Based upon the various indications provided by the various units in thesignal detection unit 240, or based upon program signals from, e.g., anexternal unit 270 under the control of a healthcare provider, one ormore impedance adjustment actions may be initiated by the IMD 200. TheIMD 200 may use data provided by the signal detection unit 240 toperform a calculation of the impedance that may be used to counter thedetected energy. This calculation may include performing a look-upfunction in a look-up table that may be stored in the memory unit 217.Those skilled in the art having benefit of the present disclosure willappreciate that other forms of signal detection may be performed andstill remain within the spirit and scope of the present invention.

Turning now to FIG. 4, a stylized depiction of the impedance unit 250 isprovided, in accordance with an illustrative embodiment of the presentinvention. The impedance unit 250 may comprise a switching controller410, a switching network 420, and an impedance array 430. The switchingcontroller 410 may comprise hardware, software, and/or firmware unitsthat are capable of controlling the switching of various impedancevalues associated with various portions of the implantable system 100.The switching controller 410, along with the switching network 420,which may comprise a plurality of switches 420, may be used to switchvarious portions of the impedance array 430.

The impedance array 430 may comprise a plurality of inductive,capacitive, resistive, and/or active components, as well as a simpleopen or short circuit. As illustrated in FIG. 4, the impedance array 430may comprise an inductive impedance L₁ in series with another inductiveimpedance L₂. The combination of the series inductive impedances L₁ andL₂ may be arranged in parallel with a plurality of inductive impedances,such as L_(n). These inductive impedances may be arranged in parallelwith a set of capacitive impedances C₁, and C₂, which are arranged inseries. The series capacitors C₁ and C₂ may be arranged in parallel witha plurality of parallel capacitive impedances, such as C_(m). Similarly,these capacitive elements may be arranged parallel related to a pair ofseries resistors R₁, R₂, which may be arranged in parallel with variousparallel resistors, such as R_(j). Also, a fixed or switchable open orshort-circuit, S₁, may be used alone, or in combination with theimpedances described above, wherein the short-circuit may also beswitched on or off by the switching network 420.

In addition to purely passive impedances, active circuitry of adequatefrequency response capability may be employed to actively reduce orsubstantially cancel coupled energy. Referring again to FIG. 2, the IMD200 may comprise an active cancellation unit 257 that is capable ofproviding an active signal to reduce coupled energy. For example, theactive cancellation unit 257 is capable of providing a controlledcurrent signal that may be used to reduce coupled energy. The activecancellation unit 257 may provide a current signal to cause theequivalent current induced by the energy to become substantially zero.The output of the active cancellation unit 257 may be set to provide a 0Amp current in the presence of the coupled energy/signal. The activecancellation unit 257 may comprise one or more controlled current supplycircuits. In one embodiment, the active cancellation unit 257 mayprovide a current signal that is capable of substantially canceling acurrent that is induced by the coupled energy/signal. Althoughillustrated in FIG. 2 as distinct from impedance unit 250, it will beappreciated that the active cancellation unit may comprise a portion ofimpedance unit 250.

Referring again to FIG. 4, the switching network 420 is capable ofswitching various portions of the components of the impedance array 430in relation to various points of the IMD 200. These points may includenodes that are coupled to the switching network, such as a node from afirst electrode E₁, on a line 412; a node from a second electrode E₂, ona line 414; a node from an n^(th) electrode E_(n), on a line 416; and acase node 418 representing the case associated with the IMD 200. Thelines 412, 414, and 416 may represent leads. Any number of impedancesmay be selected and switched by the switching network 420 to provide adesired impedance in relation to two points between any one of the nodes412, 414, 416 and/or 418. Therefore, based upon the type ofsignal/energy that is detected by the signal detection unit 240, ordetermined in advance by, e.g., a physician prior to conducting an MRIdiagnostic procedure on a patient having an implanted medical device200, the switching controller 410 may prompt the switching network 420to provide a particular impedance in relation to any portion of theimplantable system 100 where the coupled energy/signal is detected ordetermined in advance. This impedance may be selected by invoking anycombination of the components of the impedance array 430.

The impedance selected from the impedance array 430 is switched suchthat the amplitude, frequency, and/or other characteristics of thecoupled signal/energy may be brought within an acceptable level. Inother words, the presence of the coupled signal/energy is reduced byselecting particular impedances and switching them on or off between anytwo of the nodes described above. For example, the impedance array 430may be manipulated such that if a 1.5 Tesla MRI energy is detected in aportion of the implantable system 100 (e.g., a lead), the impedanceassociated with that portion may be adjusted to provide for minimalradio frequency (RF) induced heating at 64 MHz. As another example, theimpedance array 430 may be manipulated such that if a 3.0 Tesla MRIenergy is detected in a portion of the implantable system 100 (e.g., alead), the impedance associated with that portion may be adjusted toprovide for minimal RF induced heating at 128 MHz.

Turning now to FIG. 5, a schematic relating to a representativeimpedance layout associated with an illustrative embodiment of thepresent invention is provided. For the purposes of clarity ofdescription, only nodes associated with a first electrode 505 and asecond electrode 515, along with a node associated with the casing ofthe IMD 200 (node 525), are described. However, those skilled in the arthaving benefit of the present disclosure would appreciate that theschematics may include various other connections between various othernodes and remain within the spirit and scope of the present invention.

A set of impedances may be predetermined and may be switched on or offbetween various points, as illustrated in FIG. 5. For example, duringnormal operation, a normal impedance Z_(normal) 510 may be presentbetween the nodes and/or leads associated with the first and secondelectrodes 505, 515. The node 505 may represent the lead that connectsthe first electrode to the IMD 200. The node 515 may represent the leadthat connects the second electrode to the IMD 200. Together, theleads/nodes 505 and 515 may form a lead set. A switch 512 may becontrolled such that during normal operations of the IMD 200, the switch512 is closed to provide the normal impedance, Z_(normal) 510. Thenormal impedance Z_(normal) 510 may be predetermined to provide fordesirable efficiency in delivering the stimulation signal from the IMD200. However, upon detection of a significant amount of coupledsignal/energy on the node 505 or 515, or upon receiving a signal from ahealthcare provided indicating that a coupled signal may be provided inthe future, one of a plurality of impedances that may be desirable for asafe-mode operation may be switched on, while the Z_(normal) 510 isswitched off. This impedance change may be implemented such that normaloperation of the IMD 200 may continue in a safe mode, or be suspended,until the presence of the coupled energy is substantially depleted or asignal is received instructing the IMD 200 that a known coupledsignal/energy has been removed and that normal operation should resume.The safe mode may represent a mode of operation of the IMD 200 where theimpedance relating to the portion of the IMD 200 that is affected by acoupled signal/energy may be modified such that the effect(s) of thecoupled signal/energy are reduced.

During the safe mode operation of the IMD 200, a number of impedances,Z_(safe(1)) 520, Z_(safe(2)) 530, through Z_(safe(n)) 540 may beselected to provide for the attenuation of the coupled signal/energy.Each of these impedances may be respectively switched on or off in anycombination by the switches 522, 532, 542. The normal impedanceZ_(normal) 510 may be invoked or disabled by the switch 512.

As an example, for a particular coupled signal/energy that is detected(e.g., energy from a 1.5 Tesla MRI signal), it may be determined thatZ_(safe(1)) 520 is an appropriate response to substantially reduce theeffect(s) of the coupled energy. For example, the Z_(safe(1)) 520 mayprovide for a reduction of RF heating at 64 MHz. As another example, fora particular coupled signal/energy that is detected (e.g., energy from a3.0 Tesla MRI signal), it may be determined that Z_(safe(2)) 530 is anappropriate response to substantially reduce the effect of the coupledenergy. For instance, the Z_(safe(2)) 530 may provide for a reduction ofRF heating at 128 MHz. Therefore, upon such a detection, or an externalinput indicating that such a signal is expected to occur in the nearfuture, the impedance Z_(normal) 510 may be switched off by the switch512, while the impedance Z_(safe(2)) may be switched on by the switch532. Hence, during the presence of the particular coupled energy, a safeimpedance Z_(safe(2)) 530 is implemented between the nodes associatedwith the first electrode 505 and the second electrode 515. The term“safe impedance” refers to an impedance that may reduce the affects of acoupled energy. Upon termination of the event that caused the coupledenergy, the Z_(safe(2)) impedance 530 may be switched off, and theZ_(normal) 510 impedance may be switched on by the switch 512. Hence,after the presence of the coupled energy is substantially diminished,normal operations of the IMD 200 may be resumed.

Similarly, the impedance between other nodes of the implantable system100 may also be controlled. For example, the impedance between the node505 associated with the first electrode 505 and case associated with theIMD 200 on the node 525, may be altered by switching from a normalfirst-electrode-to-case impedance, Z_(normal-E1-case) 550, to anotherimpedance. The normal first-electrode-to-case impedance,Z_(normal-E1-case) 550 may represent the normal impedance that is to beimplemented between the node 505 of the first electrode 505, and thenode associated with the case 525. Upon the detection of a coupledsignal/energy between these two nodes, or on receiving a signalindicating that the impending presence of a known coupled signal/energy,the IMD 200 may switch the impedance Z_(normal-E1-case(l)) 550 off andmay implement another safe mode impedance, such as the impedanceZ_(safe-E1-case(l)) 560, or the impedance Z_(safe-E1-case(2)) 570. Thisswitching may be controlled by the switches 552, 562, and/or 572.Therefore, upon detection of the presence of a particular coupledsignal/energy, such as an MRI signal, the Z_(safe-E-case(1)) 560 may beswitched on by the switch 562 to reduce the coupled energy experiencedby a portion of the implantable system 100.

Similarly, the impedance between the node 515 associated with a secondelectrode, and the node 525 associated with the case may be modifiedbased upon a coupled signal/energy detected on at least one of these twonodes. The normal impedance Z_(normal-E2-case) 580 is the normalimpedance used during normal operation of the IMD 200. The normalimpedance Z_(normal-E2-case) 580 may be switched on or off by the switch582. Upon detection of a coupled signal/energy, the IMD 200 may switchon the Z_(safe-E2-case(1)) and/or the Z_(safe-E2-case(2)) by switchingone or more of the switches 582, 592 and/or 597. Therefore, asillustrated in FIG. 5, various impedances between various nodesassociated with the IMD and surrounding components of the implantablesystem 100 may be implemented. Those skilled in the art would appreciatethat the blocks represented by the impedances described above maycomprise the impedance array 430 of FIG. 4 in one embodiment. In analternative embodiment, the impedance blocks of FIG. 5 may comprise apredetermined set of impedances.

Turning now to FIG. 6, a flowchart associated with a method inaccordance with multiple embodiments of the present invention isprovided. The IMD 200 may perform normal operations (block 610) until asignificant amount of coupled signal/energy is detected. In oneembodiment, the IMD 200 may perform a dynamic impedance adjustmentprocess (block 640). The dynamic impedance adjustment process may callfor implementing a safe mode operation of the IMD 200. The safe modeoperation may comprise delivering stimulation while a modified impedanceis implemented. The safe mode operation may also, or alternatively,involve implementing an active cancellation of the energy, which may beperformed by the active cancellation unit 257. The safe mode may alsoinvolve suspending or reducing the delivery of therapy by the IMD 200.The safe mode implementation may be initiated by a variety of methods,such as operator input, external input, input by the patient, and thelike.

FIG. 6 also illustrates an alternative embodiment path denoted by dottedlines and dotted blocks. The alternative embodiment may call forperforming a detection of a signal/energy to initiate an implementationof the safe-mode. In this alternative embodiment, the IMD 200 mayperform a detection operation to detect if a coupled signal has beencoupled to any portion of the implantable system 100 (block 620). Forexample, the IMD 200 may perform a detection algorithm to detect thepresence or absence of coupled energy in the leads connected to the IMD200. The detection step 620 may be an ongoing or a periodic functionthat may be predetermined or may be adjusted using external inputs. Anumber of types of detecting methods may be employed, including signaldetection methods, comparison methods, thermal energy sensing methods,etc.

The IMD 200 may then make a determination whether significant coupledsignal/energy is detected (block 630). In other words, the coupledenergy that is detected may be analyzed (e.g., a comparison to apredetermined threshold) to determine whether the coupled energy shouldbe addressed. If significant coupled energy is not detected, the IMD maycontinue to perform normal operations, which may include furthercontinuous or periodic detection steps to check for the presence/absenceof coupled energy.

Upon a determination that significant coupled signal/energy has beendetected, the IMD may perform a dynamic impedance adjustment process(block 640). A more detailed description of the dynamic impedanceadjustment process of block 640, is provided in FIG. 7 and theaccompanying description below. Upon performing the dynamic impedanceadjustment process, the IMD 200 may continue normal operations (block650). Therefore, the IMD 200 may continue detecting any coupledsignal/energy and the process may be repeated.

Turning now to FIG. 7, a more detailed flowchart depiction of the stepsassociated with performing the dynamic impedance adjustment process ofblock 640 in FIG. 6 is illustrated. Upon a determination that asignificant amount of coupled signal/energy is coupled to a portion ofthe IMD 200, the IMD 200 may identify the type of coupled signal/energy(block 710). For example, a certain MRI signal with a particularfrequency and/or amplitude (e.g., 0.3 Tesla MRI signal, 0.5 Tesla MRIsignal, 0.7 Tesla MRI signal, 1.0 Tesla MRI signal, a 1.5 Tesla MRIsignal, a 3.0 Tesla MRI signal, a 5.0 Tesla MRI signal, a 7.0 Tesla MRIsignal, or the like) may be identified as the coupled energy on aportion (e.g., a lead) of the implantable system 100. Othercharacterizations of the signals may be performed to identify thecharacteristics of the coupled energy/signal or noise.

Upon a determination of the type of coupled energy that is detected, adetermination may be made as to the type of impedance that is to beimplemented to reduce the coupling effect (block 720). This may includeperforming a look-up from a look-up table to identify a particularimpedance that is to be implemented. Other input, such as manual input,or input from the external device 270, may be received in order todetermine the impedance that would cause a reduction in the coupledenergy.

The IMD 200 may also then determine the location of the coupled signal730 in order to implement the impedance (block 730). In other words, theIMD 200 may determine the safe mode impedance between a particular setof nodes to switch on or off. The safe mode may relate to continuedoperation and delivery of stimulation in predetermined intervals, albeitduring a configuration where the impedance is altered. Upondetermination of the location of the coupled signal, the impedance maybe modified based on the location of the coupled signal/energy (block740). For example, the impedance between a first electrode and a nodeassociated with the case of the IMD may be modified based on detecting acoupled signal/energy on the lead associated with the first electrode.Based upon the modification, a determination is made if the coupledsignal/energy has substantially subsided (block 750). This may bedetermined by an indication that a particular signal source has beenturned off. This indication may also be provided by an externalindication, e.g., by a physician using an external programmer unit suchas external unit 270 to indicate that a particular MRI procedure hasbeen completed. The subsiding of the coupled signal/energy may also bedetermined by detecting that the coupled energy has substantiallysubsided, i.e., by a detecting step similar to step 620 and subsequentdetermination step similar to step 630 that no significant coupledsignal/energy is present. If a determination is made that the coupledenergy has not substantially subsided, the safe mode impedance ismaintained (block 760). However, if the coupled energy or the event thatcauses such a coupled energy has subsided, then the impedances may beswitched back to normal (block 770). Therefore, the safe mode is thenterminated and a normal mode is initiated and the normal operation ofthe IMD 200 is resumed.

Utilizing the embodiments of the present invention, coupledsignal/energy may be substantially attenuated. This attenuation may beachieved by using one of a number of various impedances between variousportion(s) of the implantable system 100. Utilizing the dynamicimpedance adjustment of the present invention, a dynamic safe modeadjustment may be implemented to reduce the effects of coupled energy.For example, if a patient implanted with an IMD 200 enters an MRIchamber, the safe mode may be implemented until the MRI signals havebeen turned off to prevent adverse effects caused by the coupling of theMRI energy. Utilizing the embodiments of the present invention, adynamic response to coupled signal/energy may be performed to promote asafer and more accurate operation of implantable medical devices.Embodiments of the present invention may be implemented for a variety oftypes of implantable devices that are capable of stimulating any portionof the human body.

The particular embodiments disclosed above are illustrative only as theinvention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails of construction or design herein shown other than as describedin the claims below. It is, therefore, evident that the particularembodiments disclosed above may be altered or modified and all suchvariations are considered within the scope and spirit of the invention.Accordingly, the protection sought herein is as set forth in the claimsbelow.

What is claimed is:
 1. A method for minimizing current in a lead circuitof an implantable medical device (IMD) system induced by coupled energyfrom an external source, comprising: providing a first impedance betweena first node and a second node of the IMD, the lead circuit comprising afirst lead configured to couple the first node of the IMD to a firsttarget tissue and a second lead configured to couple the second node ofthe IMD to a second target tissue; receiving an indication of thepresence of the coupled energy on the lead circuit; determining acharacteristic of the coupled energy; selecting a second impedance froma plurality of impedances based at least in part on the characteristicof the coupled energy, the second impedance selected to minimize thecurrent induced by the coupled energy; and providing the selected secondimpedance between the first node and the second node in place of thefirst impedance, wherein the first impedance and the second impedanceare not open circuit impedances.
 2. The method of claim 1, furthercomprising: providing a stimulation signal from the IMD through the leadcircuit to deliver a therapy in the presence of the coupled energy, theIMD having the second impedance between the first node and the secondnode.
 3. The method of claim 2, wherein the stimulation signal isprovided to a vagus nerve.
 4. The method of claim 1, further comprising:determining that the coupled energy between the first node and thesecond node has substantially diminished; and providing the firstimpedance in place of the second impedance between the first node andthe second node based upon determining that the coupled energy on thelead circuit has substantially diminished.
 5. The method of claim 1,wherein the characteristic of the coupled energy comprises a magneticresonance imaging (MRI) signal level including one of 0.3 Tesla, 0.5Tesla, 0.7 Tesla, 1.0 Tesla, 1.5 Tesla, 3.0 Tesla, 5.0 Tesla, and 7.0Tesla.
 6. The method of claim 1, wherein the second impedance isselected to substantially reduce radio frequency (RF) heating of thelead circuit at one of about 128 MHz, about 64 MHz, and about 42.67 MHz.7. The method of claim 1, wherein the second impedance includes at leastone of a capacitor and an inductor.
 8. The method of claim 1, whereinreceiving the indication of the presence of the coupled energy comprisesreceiving at least one of an input from a source external to the IMD anda signal responsive to a detection of the coupled energy.
 9. The methodof claim 1, further comprising: determining that the coupled energy onthe lead circuit has changed to a second coupled energy; determining asecond characteristic of the second coupled energy; selecting a thirdimpedance from the plurality of impedances based at least in part on thesecond characteristic of the second coupled energy, the third impedanceselected to minimize current induced by the second coupled energy; andproviding the selected third impedance between the first node and thesecond node in place of the second impedance, wherein the thirdimpedance is not an open circuit impedance.
 10. The method of claim 1,wherein determining the characteristic of the coupled energy comprisesdetermining that the coupled energy is greater than a predeterminedthreshold.
 11. The method of claim 1, wherein determining thecharacteristic of the coupled energy comprises determining a frequencyof the coupled energy.
 12. The method of claim 11, wherein the secondimpedance is selected based at least in part on the frequency of thecoupled energy.
 13. The method of claim 1, wherein the plurality ofimpedances are provided by a programmable impedance array between thefirst node and the second node.
 14. A method for minimizing current in alead circuit of an implantable medical device (IMD) system induced bycoupled energy from an external source, comprising: providing a firstimpedance between a first node and a second node of the IMD, the leadcircuit comprising a first lead configured to coupled the first node ofthe IMD to a first portion of a cranial nerve and a second leadconfigured to couple the second node of the IMD to a second portion ofthe cranial nerve; detecting the coupled energy on the lead circuit at adetection unit of the IMD; determining a characteristic of the coupledenergy at the detection unit; selecting a second impedance from at leastthree impedances based at least in part on the characteristic of thecoupled energy, the second impedance selected to minimize the currentinduced by the coupled energy; providing the selected second impedancebetween the first node and the second node in place of the firstimpedance, wherein the first impedance and the second impedance are notopen circuit impedances; and providing a stimulation signal from the IMDthrough the lead circuit to deliver a therapy to the cranial nerve inthe presence of the coupled energy, the IMD having the second impedancebetween the first node and the second node.
 15. The method of claim 14,wherein the cranial nerve is a vagus nerve.
 16. The method of claim 14,further comprising: determining that the coupled energy between thefirst node and the second node has substantially diminished; andproviding the first impedance in place of the second impedance betweenthe first node and the second node based upon determining that thecoupled energy on the lead circuit has substantially diminished.
 17. Themethod of claim 14, wherein the characteristic of the coupled energycomprises a magnetic resonance imaging (MRI) signal level including oneof 0.3 Tesla, 0.5 Tesla, 0.7 Tesla, 1.0 Tesla, 1.5 Tesla, 3.0 Tesla, 5.0Tesla, and 7.0 Tesla.
 18. The method of claim 14, wherein the secondimpedance is selected to substantially reduce radio frequency (RF)heating of the lead circuit at one of about 128 MHz, about 64 MHz, andabout 42.67 MHz.
 19. The method of claim 14, wherein the secondimpedance includes at least one of a capacitor and an inductor.
 20. Themethod of claim 14, further comprising: determining that the coupledenergy on the lead circuit has changed to a second coupled energy;determining a second characteristic of the second coupled energy;selecting a third impedance from the plurality of impedances based atleast in part on the second characteristic of the second coupled energy,the third impedance selected to minimize current induced by the secondcoupled energy; and providing the selected third impedance between thefirst node and the second node in place of the second impedance, whereinthe third impedance is not an open circuit impedance.
 21. The method ofclaim 14, wherein determining the characteristic of the coupled energycomprises at least one of determining that the coupled energy is greaterthan a predetermined threshold and determining the characteristic of thecoupled energy comprises determining a frequency of the coupled energy.22. The method of claim 14, wherein the at least three impedances areprovided by a programmable impedance array between the first node andthe second node.
 23. A method for minimizing current in a lead circuitof an implantable medical device (IMD) system induced by coupled energyfrom an external source, comprising: providing a first impedance betweena first node and a second node of the IMD, the lead circuit comprising afirst lead configured to coupled the first node of the IMD to a firstportion of a cranial nerve and a second lead configured to couple thesecond node of the IMD to a second portion of the cranial nerve;detecting the coupled energy on the lead circuit at a detection unit ofthe IMD; determining a characteristic of the coupled energy at thedetection unit; selecting a second impedance from at least threeimpedances in a programmable impedance array based at least in part onthe characteristic of the coupled energy, the second impedance selectedto minimize the current induced by the coupled energy; providing theselected second impedance between the first node and the second node inplace of the first impedance, wherein the first impedance and the secondimpedance are not open circuit impedances; providing a stimulationsignal from the IMD through the lead circuit to deliver a therapy to thecranial nerve in the presence of the coupled energy, the IMD having thesecond impedance between the first node and the second node; determiningthat the coupled energy on the lead circuit has changed to a secondcoupled energy; and dynamically adjusting the second impedance tominimize a second current induced by the second coupled energy.
 24. Themethod of claim 23, wherein the second impedance includes at least oneof a capacitor and an inductor.
 25. The method of claim 23, wherein thesecond impedance is selected to substantially reduce radio frequency(RF) heating of the lead circuit at one of about 128 MHz, about 64 MHz,and about 42.67 MHz.